Field of the Invention
Embodiments of the present invention relate, in general, to accelerometers, and more particularly to a thermally balanced differential accelerometer.
Relevant Background
Accelerometers are widely used in many different application areas such as geophysical surveying, imagery, navigation, medicine, automotive, aerospace, consumer electronics and the like. These sensors detect acceleration using a fundamental application of Newton's Second Law of Motion, Force (F) is equal to Mass (m) multiplied by Acceleration (a) (F=Ma). Early accelerometer mechanisms were comprised of little more than an internal mass (m), a system that suspends the mass along an axis of interest (a sensitivity axis) and a pointer with a graduated scale to read out acceleration, typically in units of gravity (g). As the mass accelerates along the axis of sensitivity the springs are displaced. When one knows the mass of the object and the force exhibited by the displacement of the springs, he or she can determine the acceleration of the mass.
Although it has been refined and miniaturized this fundamental arrangement is still used in many of today's most advanced accelerometers. This is true even for the most widely produced class of accelerometers that are based on MicroElectroMechanical Systems (MEMS) technologies. Accelerometers of this type are used in consumer products ranging from automobiles to the latest generations of hand held telephones.
MEMS are separate and distinct from the hypothetical vision of molecular nanotechnology or molecular electronics. MEMS are made up of components that are between 1 to 100 micrometers in size (i.e. 0.001 to 0.1 mm). MEMS devices generally range in size from 20 micrometers (20 millionths of a meter) to a millimeter (i.e. 0.02 to 1.0 mm). They usually consist of a central unit that processes data (the microprocessor) and several components that interact with the surroundings such as micro-sensors. At these size scales, the standard constructs of classical physics are not always useful. For example, because MEMS have a large surface area to volume ratio, surface effects (such as electrostatics and wetting) are more dominant than volume effects (such as inertia or thermal mass).
In a typical MEMS accelerometer, as would be known to one of reasonable skill in the relevant art, the main the systems use some sort of pick-off sensor to produce an electronic signal that is proportional to acceleration. A typical arrangement of the most modern devices, such as the 3-axis accelerometer, is shown in FIG. 1. These sensors typically detect acceleration by measuring a change of position of a proof mass, for example, by a change in the associate capacitance.
In the device depicted in FIG. 1 the stationary fingers 110 are actually long thin beams used to sense the motion of the inertial mass 120 through changes in capacitance. The inertial mass 120 is suspended from a substrate by a plurality of anchors 140 using flexible legs or arms 145. As the inertial mass is displaced, one can determine the force to deform the flexible legs 145 and thereafter determined the acceleration. In this example, this displacement is measured by variance in capacitance. Each stationary finger 110 forms one plate of a variable capacitor, while the nearest protruding fingers 130 from the inertial mass 120 forms the other plate. The net capacitance for a given stationary finger is a function of the area of overlap of the two plates and the separation distance between them. For a typical MEMS type accelerometer this equates to approximately 2×10-15 farads. Therefore, in order to achieve a practical amount of capacitance it is necessary to have around forty stationary fingers arranged along each side of the inertial mass.
Measurement noise and range may vary for different applications of such sensors. Most accelerometers do not perform well at very low levels of acceleration as they are limited by inherent noise and drift in their electronics. However, as refinements occur in the ability to control and compensate noise and electronic drift, the normal inconsequential physical limitations and effects begin to dominate performance. The MEMS physical arrangement of the position sensor and inertial mass of accelerometers of the prior art typically neglects small-scale sources of error, such as thermal gradients and they lack any sort of thermal-reference plane. As a result, there are no known MEMS capacitance type accelerometers that can achieve thermal stability. One or more embodiments of the present invention address these and other deficiencies of the prior art.
Additional advantages and novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities, combinations, compositions, and methods particularly pointed out in the appended claims.